The purpose of this project is to study changes in actomyosin-like contractile proteins which accompany the aging of cultured non-muscle cells. The goal of these studies is to determine whether or not these changes can help to account for the cessation of motile behavior and growth of normal cells which accompany senescence in vitro. Since actin-like microfilaments and their associated proteins represent the major ultrastructural component of the cytoplasmic contractile machinery, their structure, localization, supramolecular organization and function are emphasized throughout the proposed research. Attempts are made to correlate function with structure through the coordinated use of microscopical, physiological, biochemical and immunological techniques.